In steel sheet products, a chromate coating has conventionally been formed on a surface of a steel sheet or a surface of an Sn, Zn, Ni or other coating formed by plating on the steel sheet in order to ensure the properties such as corrosion resistance, rust resistance and adhesion of a coating material.
In recent years, however, regulations limiting the use of hexavalent chromium have been considered with increasing interest in the environment and it is proposed to use a chemical conversion coating composed of a Zr compound as a new coating replacing the chromate coating. More specifically, a Zr-based chemical conversion coating having excellent performance can be obtained by carrying out electrolytic treatment (e.g., cathodic electrolytic treatment) in a metal surface treatment solution containing a zirconium (Zr) compound.
In the chemical conversion treatment method, successive production of a chemical conversion coating reduces the Zr ion concentration in the metal surface treatment solution containing a Zr compound. In order to solve this problem, Patent Literature 1 proposes a Zr ion-supplying method for consistently adhering a Zr-based chemical conversion coating to the surface of a steel sheet on a continuous electroplating line.
More specifically, as a result of electrolytic treatment in the metal surface treatment solution containing a Zr compound, hydrogen ions or the like are reduced in the vicinity of a cathode electrode to increase the pH of the solution in the vicinity of a steel sheet to be plated, whereby a coating of a Zr compound such as zirconium oxide is formed on the steel sheet. For instance, in a case where H2ZrF6 is used, the following reaction proceeds:H2ZrF6+2H2O→ZrO2+6HF  Formula (1)
As shown in formula (1) above, this reaction produces HF as a by-product. Since the HF is not contained in the coating, the HF remains in the metal surface treatment solution and its concentration increases. Since HF is on the right side of formula (1), an increase in the amount of HF suppresses the reaction, making it difficult for a coating to be deposited. Then, an attempt has heretofore been made to keep the HF concentration at a constant level through automatic drainage of the metal surface treatment solution. However, from an environmental and economic point of view, it was not preferable for drainage water containing large amounts of Zr ions, HF and the like to be discharged at all times.
Then, Patent Literature 1 proposes that a fluorine-free Zr compound should be used in a predetermined amount to supply Zr ions to a metal surface treatment solution so that the above-mentioned problem can be solved.